The diameter of the aperture determines the intensity of light admitted. archeology: The study of human history and prehistory through the excavation of sites and the analysis of physical remains, such as graves, tools, pottery, and other artifacts.

The pupil of a human eye is a self-adjusting aperture. archetype: The original form or body plan from which a group of organisms develops.

These all develop through an embryo that is enclosed within a membrane called an amnion.

The amnion surrounds the embryo with a watery substance, and is probably an adaptation for breeding on land.

acquired trait: A phenotypic characteristic, acquired during growth and development, that is not genetically based and therefore cannot be passed on to the next generation (for example, the large muscles of a weightlifter).